Okay,
If I understand correctly, you do get video when the system POSTs, and when you boot from a startup disk.
Try booting normally, and press F8 when the screen displays that Windows is Starting.
Select "Safe Mode"
If the system boots, then right click on a blank area of the desktop,
select properties,
click on settings,
click on apply.
Re-boot.
This will put you into Windows with a basic video setup.
Now, you need to check for correct drivers:
AGP "mini-port" driver for the motherboard / chipset.
DirectX (run dxdiag from the run box on the start menu)
Video card driver.
Monitor Driver (at least that the correct monitor is selected)
You may need to download and re-install 1 or more drivers.
If the system does not boot into safe mode, or does not boot into normal mode, I would re-install Windows.
Boot from the startup disk,
Select with CD-ROM support.
Have the W98 CD in the drive, and run setup.
You will need to re-install some updates.
Once you have Windows running again, update and run your virus scanner to make sure the problem was not caused by a virus.
Chas